The present invention relates to an ignition system for internal combustion engines which is capable of electronically controlling the ignition timing of an engine.
Ignition systems have been known in the past in which an alternating current periodically repeated at a predetermined position (e.g., the top dead center) of each cylinder of an engine is reshaped to produce a reference pulse signal whose leading edge is shifted in an ignition timing advancing direction with increase in the engine speed, so that a desired ignition timing is computed in terms of a time and determined at an elapsed time point from the leading edge of the reference pulse signal thereby controlling the ignition timing with the simple construction (e.g., Japanese Unexamined Publication No. 195867/82).
Other ignition systems have been known in which in order to prevent any erroneous operation due to a noise signal superposed on an ac signal from an electromagnetic pickup, a pump-up circuit is provided so that at times of leading-edge and trailing-edge transitions of a pulse signal generated from a waveform reshaping circuit by reshaping the ac signal, biases each increasing to the maximum just after the transition and then decreasing gradually are alternately fed back in such directions which facilitate these transitions (e.g., Japanese Unexamined Publication No. 188923/83).
However, the former of these conventional systems is disadvantageous in that since the threshold voltage of the waveform reshaping circuit for reference signal generating purposes is basically constant, it is impossible to obtain a reference pulse signal of a desired duty cycle over a range of low to high speeds from the ac signal of the signal generator whose peak value varies with the engine speed.
In the case of the latter, the ac signal from the electromagnetic pickup includes a slope portion which varies sharply in one direction and another slope portion which varies gradually in the other direction, so that during for example the engine starting period where the engine speed is extremely low, the peak value of the ac signal from the electromagnetic pickup is reduced and the biases from the pump-up circuit are also reduced thus decreasing the difference between the gradually varying portion of the ac signal and the bias from the pump-up circuit. As a result, if a noise signal is superposed on the gradually varying portion of the ac signal due to the effect of the starter or the like during the engine starting, due to the bias from the pump-up circuit which increases to the maximum immediately after the transition and then decreases gradually, the noise signal, despite it being a whisker-like noise signal, is reshaped into a pulse signal of a certain time width by the waveform reshaping circuit and this pulse signal operates the ingition coil thus causing an erroneous ignition.